1. Field of the Invention
The present invention relates to an optical sensor for measuring magnetic field intensity or current intensity on the basis of magneto-optical effect i.e., Faraday effect, and for measuring electric field intensity or voltage on the basis of electro-optical effect i.e., Pockels effect, more particularly to an optical sensor providing high sensitivity and excellent temperature characteristics in a low magnetic field range.
2. Related Art
In recent years, in accordance with progress of digital technology and computor technology, information processing technologies such as a LAN (Local Area Network) which is established by combining optical technology with the digital tecnology or the computor tecnology, has made remarkable advances. As the progress of the information processing technology, even in electric power industry, various types of optical sensors for measuring voltage or current intensity have been developed to protect and detect the fault of electric power systems such as an earth fault, a short circuit, and the breakage of cable or wire, or control of the electric power system.
Among such optical sensors, magneto-optical sensors for measuring current intensity using the Faraday effect (magneto-optical effect) and electro-optical sensors for measuring voltage using the Pockels effect have been energetically studied. The magneto-optical sensors are used for magnetic field measurement. However, when the magnetic field is induced by current, the current intensity can also be measured by the magneto-optical sensor on the basis of a relationship between current intensity and magnetic field intensity defined in the Biot-Savart law.
On the other hand, when linearly polarlized light is incident upon an electro-optical element (Pockels effect element) which is located in an electric field to be measured by the electro-optical sensor, refractive indexes with respect to orthogonal components of the linearly polarlized light are varied, so that a phase difference is imparted between the orthogonal components of the polarlized light to thereby output a retarded polarlized light. This phenomenon is called "Retardation". The electro-optical sensor is used for obtaining the electric field intensity or voltage by measuring the aforementioned phase difference of the polarlized light as a variation of the light intensity.
FIG. 6 shows a block diagram of a conventional magneto-optical sensor. The magneto-optical sensor shown in FIG. 6 comprises collimators 3a and 3b, a polarizer 4, a Faraday element (magneto-optical element) 5, analizer 6, which are linearly arranged in this order, a light source 1 connected to the collimator 3a via an optical fiber 2a, and a light receiving signal unit 7 connected to the collimator 3b via an optical fiber 2b.
A light beam irradiated from the light source 1 and having a predetermined wavelength passes through a transmission path composed of the optical fiber 2a, and reaches to the collimator 3a. Then, the collimator 3a controls a course of the light to form a paralell light flux. Thereafter, the paralell light flux is converted by the polarizer 4 into linearly polarized light.
The linearly polarized light, upon entering the Faraday element 5 located at a position where the magnetic field strength is to be measured, rotates a polarization plane in proportional to the magnetic field intensity. Transmitted light from the Faraday element 5 is then intensity modulated by the analyzer 6. Subsequently, the transmitted light from the analizer 6 is converged into a paralell light flux by the collimator 3b, and then the parallel light flux is transmitted to the light receiving signal unit 7 via the optical fiber 2b. The light receiving signal unit 7 converts the parallel light flux into an electrical signal having a level corresponding to the intensity of the parallel light flux. Therefore, the magnetic field intensity can be measured from the obtained electrical signal.
Faraday rotation angle .theta. i.e., the rotation angle .theta. of the polarization plane by the Faraday rotation effect is expressed by the following equation (1): EQU .theta.=V.multidot.H.multidot.L (1)
where V, H and L denote Verdet constant, magnetic field intensity and light path length of the Faraday element 5, respectively.
Further, a light output P.sub.OUT an intensity of which is modulated by the analizer 6 after the Faraday rotation is expressed by the following equation (2): EQU P.sub.OUT =K(1+sin 2.theta.) (2)
wherein K denotes a constant.
Accordingly, if the electrical signal converted from the light output P.sub.OUT by the light receiving signal unit 7 is measured, the magnetic field intensity in the Faraday element 5 can be obtained.
Furthermore, in the case that the magnetic field is caused by current flowing through an electric conductor, the current intensity can be similarly obtained from the electrical signal. At this time, a linear relationship exists between .theta. and P.sub.OUT in a small .theta. value range.
The representative chemical compounds providing Faraday effect are as follows: diamagnetic materials such as ZnSe having a Verdet constant V of 0.34.times.10.sup.-2 deg/Oe.multidot. cm at a wavelength of 820 nm, Bi.sub.12 SiO.sub.20 having a Verdet constant V of 0.16.times.10.sup.-2 deg/Oe.multidot.cm at a wavelength of 870 nm, Bi.sub.12 GeO.sub.20 having a Verdet constant V of 0.31.times.10.sup.-2 deg/Oe.multidot.cm at a wavelength of 850 nm, and the like.
Further, the following equation (3) is derived from the aforementioned equations (1) and (2): EQU P.sub.OUT =K(1+sin 2 V H L) (3)
Apparently, in terms of the relationship expressed in the equation (3), it is confirmed that an accuracy of the magneto-optical sensor corresponding to the intensity of the light output P.sub.OUT depends on a magnitude of the Verdet constant of the Faraday element to a great extent.
However, the Verdet constants of the conventional Faraday elements are too small to get good sensitivity. Thus, in the electric power system, sufficient sensitivity and light output can be obtained only in the high current range, i.e., in the high magnetic field range, around high power transmission lines, by the usage of the magneto-optical sensor using the conventional Faraday element.
Consequently, it is difficult to get good sensitivity and high accuracy in the low current range, i.e., in the low magnetic field range.
If the low current measurement or the low magnetic field measurement by the conventional magneto-optical sensor is required, in other words, if the increase of the light output P.sub.out in the low current range or in the low magnetic field range is needed, it is required to increase the optical path length L of the Faraday element, as is evident from the relationship expressed in equation (3). However, in the case that the magneto-optical sensor having a quite long Faraday element is used in the high-current range or in the high-magnetic field range, the value of .theta. becomes so high that the linear relationship between the modulation degree M of the light intensity, which is equal to the value of Ksin2.theta. expressed in equation (2), and the value of .theta. is deteriorated, that is, the measurement error is increased.
Furthermore, the enlargement of the optical path length of the Faraday element also results in the deterioration of a feature, i.e., miniaturization which is originally considered to be the most excellent feature of this type of the magneto-optical sensor.
On the other hand, there has been a case where a rare earth iron garnet is used as a material for the Faraday element. The typical example thereof may include Y.sub.3 Fe.sub.5 O.sub.12, so-called, YIG (yttria-indium-garnet) crystal which has a Verdet constant of 0.11 deg/Oe.multidot.cm with respect to a light having a wavelength of 1300 nm which is in an infrared region. The rare earth-iron-garnet has a high Verdet constant, so that it is possible to perform the measurement with high accuracy even under the conditions of low-current or low-magnetic field.
However, the Verdet constant of the rare earth-iron-garnet disadvantageously varies widely depending on temperature variation. For example, the Verdet constant thereof varies from -8% to +12% in a temperature range of -20.degree. C. to +80.degree. C. Namely, a temperature characteristic (temperature dependence) of the Faraday element is inferior indeed. Accordingly, when the magneto-optical sensor having such Faraday element composed of rare earth-iron-garnet is used in the electric power system, the measuring operation is usually performed in out-door field, so that the Faraday element will be subjected to a large temperature variation due to a weather condition or solar radiation. Therefore, it is difficult to measure a magnetic field intensity with high accuracy, and also to adapt the sensor to a practical use.
On the other hand, as another Faraday element composed of rare earth-iron-garnet crystal of which Verdet constant is improved in temperature dependence, there has been known that one composed of Y.sub.3-x Tb.sub.x Fe.sub.5 O.sub.12 (0.3.ltoreq..times..ltoreq.0.8) containing terbium element (Tb). It has been known that the most effective and preferable wavelength for such Faraday element to realize an excellent light transmission property is a range of 1.1 to 2.0 .mu.m. As an example of the temperature characteristic of the Faraday element being operated in aforementioned wavelength range and the temperature range of -20.degree. C. to +80.degree. C., there has been found a Faraday element in which temperature variation of the Verdet constant is .+-.1 to .+-.6% or less at the wavelength of 1.15 .mu.m.
In spite of this fact, when the aforementioned Faraday element is actually used in the magneto-optical sensor, however, an absolute value of the Verdet constant at a wavelength of 1.15 .mu.m still remains at about 0.12 to 0.20 deg/Oe.multidot.cm. Therefore, in order to increase the Faraday rotation angle in a low-current or low-magnetic field condition and to obtain a suffucient light output, there is no way other than increasing the optical path length (thickness) L of the Faraday element as easiliy understood from equation (1).
As the result, when the aforementioned rare earth-iron-garnet crystal is to be adapted for forming the practical Faraday element, a film thickness i.e., the thickness of the Faraday element is required to be from 280 .mu.m to 1 mm or more.
However, in a LPE (Liquid Phase Epitaxial) method which is now considered to be the most effective or available method for manufacturing of this type of the thin film, when the film thickness for the Faraday element exceeds 250 .mu.m, defectives such as camber and crack or the like disadvantageously occur in the thin film during the epitaxial growth of the crystal at the high temperature. This is because that there may exist a large difference between a lattice constant (12.383 .ANG.) of a GGG (gadolinium-gallium-garnet) substrate which is generally used as a growth substrate for forming the thin film and a lattice constant (12.388 .ANG. in case of x=0.6) of Y.sub.3-x Tb.sub.x Fe.sub.5 O.sub.12, and because a thermal expansion coefficient of Y.sub.3-x Tb.sub.x Fe.sub.5 O.sub.12 is larger than that of GGG substrate.
On the other hand, in a case where the thin film to be used for the Faraday element is manufactured by utilizing a flux method, the thin film is produced in a bulk-form, so that the aforementioned problems occur in the LPE method can be avoided. However, crystalization properties of a product are inferior, so that it takes many hours to form the product into a film form, thus resulting in deterioration of mass-productivity of the thin film.
Further, as has been already described in the case of the Faraday element composed of diamagnetic material, when the film thickness is largely set to from 280 .mu.m to 1 mm or more, such a thick film will obstruct miniaturization of the outstanding magneto-optical sensor. In addition, in a case where the magnetic field intensity or current intensity becomes high, such a thick film also results to deteriorate the linear relationship between the modulating degree M of the light intensity and the Faraday rotation angle .theta..
Furthermore, there is posed another problem that the magneto-optical sensor having a Faraday element composed of Y.sub.3-x Tb.sub.x Fe.sub.5 O.sub.12 (0.3.ltoreq..times..ltoreq.0.8) is available for measuring low-magnetic field and low-current, however, a value of saturated magnetic field of the Faraday element is about 1400 to 1800 oersted. Therefore, when a measuring operation is performed in high-magnetic field or high-current such as to exceed 10000 oersted, the Faraday rotation angle of the element is saturated, so that it becomes impossible to perform the measuring operation. Accordingly, as is the same manner as in case of the aforementioned Faraday element composed of diamagnetic material which is able to measure only high-magnetic field and high-current, there may be difficulties such that the magneto-optical sensor cannot be applied to the electric power system wherein a wide range of magnetic field strength and variable current are involved with a high possibility.
On the other hand, a waveband of the linear polarlized light to be applied to the conventional magneto-optical sensor for measuring the aforementioned current intensity is quite different from that of the electro-optical sensor, so that it is impossible to commonly utilize a single signal processing system, and to miniaturize a size of the sensor. Therefore, the aforementioned two kinds of optical sensors such as the conventional magneto-optical sensor and the electro-optical sensor are separately manufactured as an independent sensor equipment, respectively. Accordingly, it is impossible to measure both current and voltage intensity by utilizing a single miniaturized sensor at the same measuring time.